Default:USB versus PS/2
Many people want to know whether to buy or use a PS/2 or a USB keyboard. Most modern keyboards ship default as USB. You can add a passive adapter to a handful of keyboards like the Das Keyboard and Filco to support PS/2. This is known as a passthrough adapter since all it does is cross wires/traces to the appropriate connections and is generally purple (matching the PS/2 color coding). There are some gaming keyboards like the Xarmor that do it the other way and let you add a passive adapter to go from PS/2 to USB connector. And finally, you can add an active (has electronics to emulate a USB keyboard) adapters to go from PS/2 to USB (but not the other way around).
So what to do? Should you only buy keyboards that have a PS/2 option? If your keyboard supplies both connections which to use? It pretty much boils down to convenience versus NKRO.
PS/2 Connector slowly disappearing
Nowadays the USB connector is ubiquitous whereas the PS/2 connector is fast disappearing. Long ago the PS/2 connector was omitted from laptops, and more and more desktop systems do not have PS/2 connectors (Dell/HP no longer ship with PS/2 ports). The lack of new PS/2 keyboards being designed and produced will probably end up with PS/2 going the way of Serial Ports and Parallel Ports.
Intel Gaming Motherboard
Gigabyte Gaming Motherboard - dual purpose PS/2 port
Asus Rampage III - single PS/2 keyboard port
The USB interface supports hot swapping meaning you can disconnect and reconnect devices while the system is powered on. The PS/2 connector does not technically support hot swapping, if you connect or disconnect a PS/2 device you may be forced to reboot. That said, most newer computers are tolerant of this and continue to work. But even so there are sometimes issues. I have seen computers that allowed you to hot swap PS/2 keyboards, but the typematic rate will not work correctly until rebooted.
Another benefit to the USB connector is it is easier to connect when you can not see the plug (reaching around behind a computer). The PS/2 connector being round is harder to connect without bending the pins. Also, many computers have USB connectors on the front for convenience.
USB Allows for multiple keyboards at one time
This is convenient for keyboard co-op games or if you just like having multiple keyboards to swap for different tasks (gaming versus typing).
PS/2 implemented poorly on many modern motherboards
As a result of cost cutting there are monthly reports of Geekhack user not being able to plug in keyboards. This is a typical "My Model M Doesn't Work" one. Also effects modern keyboards like the Das S as seen in this OCN post. Although not confirmed, it's probably due to power limitations on the way PS/2 is implemented. Motherboard manufacturers probably don't bother adhering to the original 1987 IBM PS/2 specification to the letter.
Macro Programming Software
Many modern gaming keyboards come with macro programming software. Because of limitations in PS/2 these vendor specific macro software programs (think Logitech and SetPoint) inevitably only work on their USB keyboards. For example Steelseries has a Macro programming routine for the SHIFT keyboard that can not be ported to the 6GV2 in PS/2 mode. Razer also claims the same thing in their Black Widow FAQ.
Quite simply it is getting harder and harder to FIND a PS/2 keyboard as Logitech/Microsoft/HP/Dell move to exclusively USB keyboards. And if you want a Realforce, a Happy Hacking, or a Razer Mechanical there IS no PS/2 choice or ability to work with a simple purple PS/2 adapter.
USB keyboards are more popular and therefore easier to sell. Unless you have a IBM M15 of course.
USB is limited to 6 standard keys plus modifiers with NKRO support, but PS/2 can support more. However, just because you are using one connector or the other does not mean you will get full NKRO support. Your keyboard must also support NKRO. See the Nkey Keyboard Listing Wiki for details. And then think about your gaming patterns. If you game with one hand on the mouse and one on the keyboard you really don't need more than 6 keys. In addition most apps and Windows will start limiting you to 12 keys so full NKRO is more of a concept than a reality.
Older BIOSes or technical work
If you have an older BIOS it may not support USB at boot. However we are talking OLD motherboards here. Most newer BIOS have "Legacy USB keyboard" support turned on by default. If you do a lot of tech work it makes sense to have a spare PS/2 keyboard in the toolkit. If you overclock heavily you may find it easier to access the BIOS through a PS/2 port if USB becomes unstable.
Pretty Much Even
Polled versus Interrupt
The USB interface is polled meaning that the computer on a regular basis checks the keyboard for key presses / releases. Up to 7 keystrokes (modifier plus characters) is put into a packet and sent when using the standard USB HID (Human Interface Device) driver. The PS/2 interface on the other hand is interrupt driven, meaning only when a key is pressed / released does the keyboard send a signal to the computer. Although this SOUNDS good the practical effect is negligible due to the overhead of the OS and applications. Keyboard marketing departments like Steelseries would like you to believe this. I recommend you test for yourself but just swapping keyboards. You'll STILL see the keys lagging periodically under PS/2. And anyway it can't be all that bad since even SteelSeries latest gaming keyboard the SHIFT is USB only.
PS/2 Hotswap anyway - screw the spec!
People hotswap all the time. If you don't swap keyboards and mice and stick to keyboard swapping it often works although you may have to rescan in Windows Device Manager. And if the PS/2 port gets fried, well, there is always USB.
Which should I use?
Unless you need more than 6KRO (plus Shift, CTRL, ALT, WIN) USB is a lot more convenient and you shouldn't limit your keyboard choices because of this factor (the Razer Black Widow for example is USB only). If you have certain games that need it or want the bragging rights of full NKRO go with PS/2. I would still get a "Blue Cube" or Belkin USB adapter just to future proof your PS/2 keyboard and throw in a drawer. Someday you'll need it.
Links To Further Information
USB versus PS/2 GeekHack Poll
Geekhack Discussion One
Geekhack Discussion Two
OCN Discussion Thread 1
OCN Discussion Thread 2
How to go into BIOS and reset if your computer isn't coming out of standby using USB
Using Windows Device Manager to locate HID entry
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